Use Eclipse Foundation OpenID Connect as the only authentication provider

OpenVSX currently requires users to log in with GitHub. In addition, users must connect their account to Eclipse Foundation OpenID Connect to sign the publisher agreement before they can publish an extension.

This means we effectively require both GitHub and Eclipse authentication for publishers.

Since an Eclipse account is mandatory for publishing, I would like to propose we simplify the process and use Eclipse Foundation OpenID Connect as the only authentication provider.

This would:

- Remove the need for dual account linking.
- Simplify the login and publishing flow.
- Reduce external dependencies in the authentication path.

We should review the technical and migration impact for existing users before proceeding.